In recent years, the Internet Engineering Task Force (IETF) has actively developed an IPv6 routing protocol for low power and lossy networks (RPL) as a communication scheme dedicated for a sensor network.
The RPL is a protocol that automatically forms an optimum route used for a communication channel based on wireless propagation routes between nodes connected in a meshed shape (network shape). Communication data is transmitted to reach a destination along a multi-hop route in a bucket relay scheme along an optimum route derived by the RPL by each node. In general, since a radio wave reach distance is physically restricted due to an antenna performance, a radio wave output value, an obstacle object, or the like, the communication range of a wireless station is restricted to a neighboring node with which the wireless station can directly communicate. However, in a wireless meshed network scheme, each node actions as a relay station of the neighboring nodes. Therefore, it is possible to obtain the characteristics in which communication with a wide-range node regardless of restriction on the radio wave reach distance is enabled.
In the RPL, priorities (ranking value) of all routes that become selection candidates are calculated based on a determination criterion (metric) defined in advance. The higher the priority of a route is, the smaller a calculated ranking value is. Therefore, the route with the smallest ranking value is selected as the optimum route. A criterion utilized as the metric has currently been discussed. As the candidates of the criterion, the number of hops indicating a meshed configuration scale, a signal intensity and a packet retransmission ratio each indicating a wireless propagation route quality, a transmission rate and a sleep state each indicating availability of a relay node, and the like have been enumerated.
In general, when the quality of a wireless propagation route is comprehended, combination of both the signal intensity and the packet retransmission ratio is recommended to improve reliability. As a specific method, a method of periodically performing a reachability test on nodes on a route and actively monitoring the signal intensity and the packet retransmission ratio has been generalized.
As described above, the method of periodically performing a reachability test on nodes on a route and actively monitoring the signal intensity and the packet retransmission ratio has been generalized to comprehend to the quality of a wireless propagation route. However, when the reachability test is introduced, the amount of control packet increases and consumption of wireless resources may thus increase. In particular, in a large-scale meshed network environment, it is necessary to perform confirming reachability to nodes on all of the routes which become alternative candidates. Further, in wireless communication at a narrow portable region at which a 900 MHz band or the like is used, an available band is short and a negative effect may be serious.